Indirect image encryption method based on polarization multiplexing metasurface

Abstract

According to the indirect image encryption method based on the polarization multiplexing metasurface, the used phase gradient metasurface is composed of nanometer bricks with singular point characteristics and mirror image structures of the nanometer bricks, when left-hand circularly polarized light and right-hand circularly polarized light are incident, the corresponding structures of the nanometer bricks play a role respectively, two holograms are constructed in a far field, and polarization multiplexing is achieved; on the basis that two structures are arranged in the column direction to form a super unit, when x-ray polarized light is incident, light is subjected to non-abnormal reflection through the generalized Snell law, required RCP and LCP are filtered out, and emergent light is superposed to generate vector light; performing condition limitation on the azimuth angle of the vector light through a simulated annealing algorithm to obtain two encrypted patterns; the encrypted pattern needs to be obtained by analyzing the polarization of the emergent light to obtain a corresponding Stokes parameter and then constructing a decryption function related to the Stokes parameter for calculation and analysis; therefore, the metasurface for realizing polarization multiplexing and indirect encryption functions of incident light in different polarization states is constructed.

Description

Technical Field

[0001] The present invention relates to the technical field of micro-nano photonics, and particularly to an indirect image encryption method based on a polarization multiplexing metasurface. Background Art

[0002] With the development of information technology, information security has become increasingly important, and various information encryption technologies have emerged. Among them, the optical image encryption technology combined with metasurfaces has been widely studied. Currently, most encryption schemes first construct different information multiplexing channels by changing the wavelength, polarization, incident direction, etc. of incident light, and then encode the image information into the corresponding channels. However, the stored images can be obtained by directly irradiating the metasurface, and the encryption difficulty of image information is low. Summary of the Invention

[0003] The present invention proposes an indirect image encryption method based on a polarization multiplexing metasurface for the above problems. Its concept lies in an indirect image encryption method based on a polarization multiplexing metasurface. By using a non-Hermitian chiral structure and its mirror structure, and restricting the rotation angle difference through a simulated annealing algorithm, a design system and method for far-field right-handed circularly polarized light (RCP) and left-handed circularly polarized light (LCP) holograms and two other vector encryption maps are realized. When incident with RCP and LCP circularly polarized light, the corresponding LCP and RCP holograms are emitted in the far field. When incident with x-polarized light, vector light is emitted, and the emitted vector light is calculated and decoded to retrieve the other two encryption maps.

[0004] The indirect image encryption method based on a polarization multiplexing metasurface includes the following steps:

[0005] (1) Construct a nano-unit structure, where the nano-unit structure includes a substrate and nano-bricks on the working surface of the substrate;

[0006] (2) Optimize the nano-unit structure to obtain a phase-gradient metasurface;

[0007] (3) Incident x-polarized light on the phase-gradient metasurface to generate vector light;

[0008] (4) Calculate the corresponding Stokes parameters according to the ellipticity angle and azimuth angle of the emitted polarization state of the vector light generated in step (3), perform zone coding on the S1 and S2 parameters in the Stokes parameters, and achieve the encryption effect through encoding design of the azimuth angle, that is, the S1 and S2 parameters;

[0009] (5) Optimize and limit the rotation angle by a simulated annealing algorithm.

[0010] More specifically, in step (1), the substrate is a silica-aluminum material, and the nano-bricks are all aluminum materials.

[0011] More specifically, step (2) includes: scanning the size range of the nanobricks in the nano-unit structure to obtain the corresponding phase and reflectivity maps, and calculating its eigenvalues to obtain the singular point structure and the mirror structure of the singular point structure;

[0012] Respectively use the singular point structure and the mirror structure to change the rotation angle of the incident circularly polarized light, and verify that the singular point structure and the mirror structure conform to the geometric phase principle and the singular point characteristic phenomenon; arrange the singular point structure and the mirror structure vertically to form a phase gradient metasurface; according to the generalized Snell's law, filter out the cross-polarized components and discard the co-polarized components, so that the singular point structure performs polarization conversion on the right-handed circularly polarized light, and the mirror structure performs polarization conversion on the left-handed circularly polarized light;

[0013] More specifically, step (3) includes: incidenting x-linearly polarized light on the phase gradient metasurface, and the phase gradient metasurface decomposes the x-linearly polarized light into right-handed circularly polarized light and left-handed circularly polarized light, and the phase gradient metasurface superimposes the right-handed circularly polarized light and the left-handed circularly polarized light to generate vector light;

[0014] The exit angle of the vector light is

[0015]

[0016] where λ is the wavelength of the incident light, θ td is the exit angle, and δ d is the rotation angle;

[0017] The exit polarization state of the vector light is:

[0018]

[0019] δ L is the rotation angle corresponding to the singular point structure, and δ R is the rotation angle corresponding to the mirror structure, A R is the exit amplitude of the singular point structure, and A L is the exit amplitude of the mirror structure;

[0020] Step (4) includes: calculating the corresponding Stokes parameters according to the ellipticity angle and azimuth angle of the exit polarization state of the vector light generated in step (3), encoding the region where S1>0 and S2>0 as "11", the region where S1<0 and S2<0 as "00", the region where S1>0 and S2<0 as "10", and the region where S1<0 and S2>0 as "01", then obtaining the corresponding two encrypted maps;

[0021] The azimuth angle ψ and ellipticity χ corresponding to the exit polarization state of the vector light are respectively:

[0022] ψ = δ R + δ L

[0023]

[0024] Stokes parameter calculation method:

[0025] S0 = I

[0026] S1 = I cos(2ψ)cos(2X)

[0027] S2 = I sin(2ψ)cos(2X)

[0028] S3 = I sin(2X)

[0029] Among them, S0 is the total light intensity, S1 is the light intensity difference between linearly polarized lights in the x and y directions, S2 represents the light intensity difference between linearly polarized lights at ±45°, and S3 represents the light intensity difference between right-handed and left-handed circularly polarized lights;

[0030] Step (5) includes: changing the rotation angles of the singular point structure and the mirror structure according to the set hologram and encryption pattern, and at the same time restricting the rotation angle ranges of the singular point structure and the mirror structure to prevent crosstalk between right-handed circularly polarized light and left-handed circularly polarized light;

[0031] When incident with externally incident right-handed circularly polarized light, a preset hologram is obtained through the Fourier iterative algorithm; when incident with externally incident left-handed circularly polarized light, another preset hologram is obtained through the Fourier iterative algorithm;

[0032] When incident with externally incident linearly polarized light in the x direction, the Stokes parameters of the outgoing light are measured, and two independent encrypted patterns are obtained through calculation and decoding.

[0033] The array size of the phase gradient metasurface is 100x100, and the size of the encryption pattern is 90x90.

[0034] The phase gradient metasurface used in the present invention is composed of nanobricks with singular point characteristics and their mirror structures. When incident with left-handed circularly polarized light and right-handed circularly polarized light, their corresponding structures act respectively, and two holograms are constructed in the far field to achieve polarization multiplexing; on the basis that two structural columns are arranged in series to form a supercell, when incident with linearly polarized light in the x direction, the light is non-anomalously reflected through the generalized Snell's law, filtering out the required RCP and LCP, and the outgoing light is superimposed to generate vector light; the azimuth angle of the vector light is conditionally restricted through the simulated annealing algorithm to obtain two encrypted patterns; and the encrypted patterns need to be obtained by analyzing the outgoing light through polarization analysis, obtaining the corresponding Stokes parameters, and constructing a decryption function related thereto; thus, a metasurface that realizes polarization multiplexing and indirect encryption functions when incident with lights of different polarization states is constructed.

[0035] The beneficial effects produced by the present invention are as follows:

[0036] 1) The technical solution of the present invention adopts a non-Hermitian chiral structure and its mirror structure. Utilizing the characteristics of its singular points, while filtering out the co-polarized components, the right-handed circularly polarized light and the left-handed circularly polarized light act independently, reducing the design steps of the metal grating, and only a single-layer nanobrick structure is required to achieve information concealment. The metasurface designed by the present invention is small in size, light in weight, highly integrable, and has a simple manufacturing process.

[0037] 2) The present invention designs two far-field holograms by utilizing the characteristics of singular points, and reduces the relevant crosstalk by using the generalized Snell's law.

[0038] 3) In the design scheme of the present invention, two encryption channels are designed based on two far-field holograms, and secondary encryption is performed. After detecting the polarization state of the outgoing light, that is, the S parameter, it is calculated and decoded. Compared with directly obtaining the plaintext image information from the nanostructure unit, the decryption difficulty and information concealment are increased. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 FIG. is a schematic diagram of the unit structure of the phase gradient metasurface used in an indirect image encryption method based on polarization multiplexing metasurface in Embodiment 1 of the present invention.

[0040] Figure 2a FIG. is a phase and reflectivity diagram of the singular point structure of the present invention.

[0041] Figure 2b FIG. is a phase and reflectivity diagram of the mirror structure of the present invention.

[0042] Figure 3 FIG. is a schematic diagram of the supercell designed by the present invention.

[0043] Figure 4a FIG. is a schematic diagram of the zonal coding of the S1 and S2 parameters in the Stokes parameters by the present invention.

[0044] Figure 4b FIG. is a schematic diagram of the principle of zonal coding by the present invention.

[0045] Figure 5 FIG. is the hologram and encryption diagram designed by the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0046] Hereinafter, the technical solution of the present invention will be further described with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only for explaining and illustrating the embodiments of the present invention, and are not used to limit the embodiments of the present invention.

[0047] An indirect image encryption method based on polarization multiplexing metasurface includes the following steps:

[0048] (1) Construct a nano-unit structure, which includes a substrate and nano-bricks on the working surface of the substrate; as Figure 1 shown, the nano-unit structure of this embodiment consists of an "L"-shaped nano-brick structure designed with Al material and an SiO2-Al-based bottom layer. First, optimize and construct the nano-unit structure according to the EP points. Scan the nano-bricks through electromagnetic simulation software to obtain a working wavelength of λ = 600 nm, and the size parameters are: L1 = 52 nm, L2 = 140 nm, L3 = 119 nm, W = 50 nm, G = 70 nm, the height is h = 30 nm, the substrate period of the unit structure is P = 300 nm, and the heights are h1 = 40 nm and h2 = 150 nm.

[0049] (2) Optimize the nano-unit structure to obtain a phase-gradient metasurface: Scan the size range of the nano-bricks in the nano-unit structure to obtain the corresponding phase and reflectivity diagrams, and calculate its eigenvalues to obtain the singular point structure and the mirror structure of the singular point structure;

[0050] Use the singular point structure and the mirror structure to change the rotation angle of the incident circularly polarized light respectively, and verify that the singular point structure and the mirror structure conform to the geometric phase principle and the singular point characteristic phenomenon; Arrange the singular point structure and the mirror structure vertically to form a phase-gradient metasurface; According to the generalized Snell's law, filter out the cross-polarized components and discard the co-polarized components, so that the singular point structure can perform polarization conversion on the right-handed circularly polarized light, and the mirror structure can perform polarization conversion on the left-handed circularly polarized light;

[0051] As Figure 2a and Figure 2b shown, in this embodiment, for the singular point structure at the incident working wavelength of 600 nm, the reflectivity of R 21 is higher than 60%, and the reflectivity of R 12 is close to 0%, and the mirror structure is the opposite; and the relationship between the rotation angle and the phase change conforms to the PB phase modulation principle and meets the requirements of the singular point characteristics.

[0052] (3) Incident x-polarized light on the phase-gradient metasurface. The phase-gradient metasurface decomposes the x-polarized light into right-handed circularly polarized light and left-handed circularly polarized light, and the phase-gradient metasurface superimposes the right-handed circularly polarized light and the left-handed circularly polarized light to generate vector light;

[0053] The exit angle of the vector light is

[0054]

[0055] where λ is the wavelength of the incident light, θ td is the exit angle, and δ d is the rotation angle;

[0056] The exit polarization state of the vector light is:

[0057]

[0058] As Figure 3 shown, δ L is the rotation angle corresponding to the singular point structure, controlling the output of right-handed circularly polarized light; δ R is the rotation angle corresponding to the mirror structure, controlling the output of left-handed circularly polarized light; the specific rotation angle is obtained after optimization by the simulated annealing algorithm. δ d is the rotation angle of the phase gradient, obtained according to the formula δ d = 45°, and the output angle is 30°. A R is the output amplitude of the singular point structure, and A L is the output amplitude of the mirror structure;

[0059] (4) Calculate the corresponding Stokes parameters according to the ellipticity angle and azimuth angle of the output polarization state of the vector light generated in step (3), and perform zonal coding on the S1 and S2 parameters in the Stokes parameters. As Figure 4a and Figure 4b shown, the region where S1>0 and S2>0 is encoded as "11", the region where S1<0 and S2<0 is encoded as "00", the region where S1>0 and S2<0 is encoded as "10", and the region where S1<0 and S2>0 is encoded as "01", then two corresponding encrypted images are obtained;

[0060] The azimuth angle ψ and ellipticity χ corresponding to the output polarization state of the vector light are respectively:

[0061] ψ = δ R + δ L

[0062]

[0063] Stokes parameter calculation method:

[0064] S0 = I

[0065] S1 = I cos(2ψ)cos(2χ)

[0066] S2 = I sin(2ψ)cos(2χ)

[0067] S3 = I sin(2χ)

[0068] where S0 is the total light intensity, S1 is the difference in light intensity between x and y linearly polarized lights, S2 represents the difference in light intensity between ±45° linearly polarized lights, and S3 represents the difference in light intensity between left and right circularly polarized lights;

[0069] (5) Optimize the rotation angle by simulated annealing algorithm: Change the rotation angles of the singularity structure and the mirror structure according to the set hologram and encryption pattern, and at the same time limit the rotation angle ranges of the singularity structure and the mirror structure to avoid crosstalk between right-handed circularly polarized light and left-handed circularly polarized light;

[0070] According to Figure 5 , when incident with external right-handed circularly polarized light, a preset hologram A is obtained through the Fourier iterative algorithm; when incident with external left-handed circularly polarized light, another preset hologram B is obtained through the Fourier iterative algorithm;

[0071] When incident with external x-polarized light, the Stokes parameters of the outgoing light are measured, and two independent encrypted patterns C and D are obtained by calculation and decoding.

[0072] The array size of the phase gradient metasurface is 100x100, and the encryption pattern size is 90x90.

[0073] The content described in this specification is only a list of the implementation forms of the inventive concept. The protection scope of the present invention should not be regarded as limited to the specific forms stated in the embodiments. The protection scope of the present invention also extends to equivalent technical means that can be conceived by those skilled in the art according to the inventive concept.

Claims

1. An indirect image encryption method based on polarization multiplexing metasurface, the steps of which include: (1) Construct a nano-unit structure, which includes a substrate and nano-bricks on the working surface of the substrate; (2) Optimize the nano-unit structure to obtain a phase gradient metasurface: (3) Incident x-linearly polarized light on the phase gradient metasurface to generate vector light; (4) Calculate the corresponding Stokes parameters according to the ellipticity angle and azimuth angle of the output polarization state of the vector light generated in step (3), perform zoning encoding on the S1 and S2 parameters in the Stokes parameters, and achieve the encryption effect by encoding and designing the azimuth angle, that is, the S1 and S2 parameters; (5) Optimize the restricted rotation angle by the simulated annealing algorithm.

2. The indirect image encryption method based on polarization multiplexed metasurface according to claim 1, wherein: In step (1), the substrate is made of silica-aluminum material, and the nano-bricks are all made of aluminum material.

3. An indirect image encryption method based on polarization multiplexing metasurface according to claim 1, characterized in that: Step (2) includes: scanning the size range of the nano-bricks in the nano-unit structure to obtain the corresponding phase and reflectivity maps, and calculating its eigenvalues to obtain the singular point structure and the mirror structure of the singular point structure; Use the singular point structure and the mirror structure to change the rotation angle of the incident circularly polarized light respectively, and verify that the singular point structure and the mirror structure conform to the geometric phase principle and the singular point characteristic phenomenon; arrange the singular point structure and the mirror structure vertically to form a phase gradient metasurface; according to the generalized Snell's law, filter out the cross-polarization components and discard the co-polarization components, so that the singular point structure performs polarization conversion on the right-handed circularly polarized light, and the mirror structure performs polarization conversion on the left-handed circularly polarized light.

4. An indirect image encryption method based on polarization multiplexing metasurface according to claim 1, characterized in that: Step (3) includes: incident x-linearly polarized light on the phase gradient metasurface, the phase gradient metasurface decomposes the x-linearly polarized light into right-handed circularly polarized light and left-handed circularly polarized light, and the phase gradient metasurface superimposes the right-handed circularly polarized light and the left-handed circularly polarized light to generate vector light; The output angle of the vector light is where λ is the wavelength of the incident light, θ td is the exit angle, and δ d is the rotation angle; The output polarization state of the vector light is: δ L is the rotation angle corresponding to the singular point structure, δ R is the rotation angle corresponding to the mirror structure, A R is the outgoing amplitude of the singular point structure, A L is the outgoing amplitude of the mirror structure.

5. An indirect image encryption method based on polarization multiplexing metasurface according to claim 1, characterized in that: Step (4) includes: calculate the corresponding Stokes parameters according to the ellipticity angle and azimuth angle of the output polarization state of the vector light generated in step (3), encode the region where S1>0 and S2>0 as "11", the region where S1<0 and S2<0 as "00", the region where S1>0 and S2<0 as "10", and the region where S1<0 and S2>0 as "01", then two corresponding encrypted images are obtained; The azimuth angle ψ and ellipticity χ corresponding to the output polarization state of the vector light are respectively: ψ = δ R + δ L Stokes parameter calculation method: S0 = I S1 = I cos(2ψ)cos(2χ) S2 = I sin(2ψ)cos(2χ) S3 = I sin(2χ) Where S0 is the total light intensity, S1 is the light intensity difference between x and y linearly polarized lights, S2 represents the light intensity difference between ±45° linearly polarized lights, and S3 represents the light intensity difference between left and right circularly polarized lights.

6. An indirect image encryption method based on a polarization multiplexed metasurface according to claim 1, characterized in that: Step (5) includes: changing the rotation angles of the singularity structure and the mirror structure according to the set hologram and encryption map, and at the same time restricting the rotation angle ranges of the singularity structure and the mirror structure to avoid crosstalk between right-handed circularly polarized light and left-handed circularly polarized light; When incident with externally incident right-handed circularly polarized light, a preset hologram is obtained through the Fourier iterative algorithm; when incident with externally incident left-handed circularly polarized light, another preset hologram is obtained through the Fourier iterative algorithm; When incident with externally incident x-polarized light, the Stokes parameters of the outgoing light are measured and two independent encrypted patterns are obtained through calculation and decoding.

7. An indirect image encryption method based on a polarization multiplexed metasurface according to claim 1, characterized in that: The array size of the phase gradient metasurface is 100x100, and the size of the encryption map is 90x90.

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